Enterobacter species generally ferment lactose promptly and produce colonies similar tothose of Klebsiella, although not as mucoid. A differential feature is motility by peritric-hous flagella, which are generally present in Enterobacter species but uniformly absent in Klebsiella. Enterobacter species, which appear to be less virulent than Klebsiella, areusually found in mixed infections, in which their significance must be decided on clinical and epidemiologic grounds. Several hospital outbreaks traced to contaminated parenteral fluid solutions have implicated Enterobacter species. In addition to ampicillin, most iso-lates are resistant to first-generation cephalosporins, but may be susceptible to second- or third-generation cephalosporins; however, mutants derepressed for β-lactamase produc-tion occur at relatively high frequency and confer resistance to many cephalosporins.